Early- and later-stage priming effects induced by spruce root fractions are regulated by substrate availability, stoichiometry and C input

The priming effect (PE), referring to the change in the soil organic carbon (SOC) turnover induced by fresh C input, is crucial to the SOC budget. Temperate coniferous forests store a large SOC pool that can be influenced by litter C input. Root litter is a major source of belowground C input but much less studied than leaf litter. Nowadays, it is not clear how PEs are controlled by root litter input of different availability and stoichiometry in coniferous forests and what the underlying mechanisms are. We prepared soluble fraction (SF) and insoluble fraction (IF) from 13C-depleted spruce roots using hot-water extraction and incubated these fractions with spruce forest soils (Cambisols) to measure the PE. Labile and slow C pools were simulated using a first-order parallel model, whereas soils were harvested during and after the incubation to estimate microbial utilization of substrate-C and C use efficiency (CUE). The SF had higher substrate availability, greater C:N ratio, and smaller C quantity than the IF. The PE ranged from -0.71 & PLUSMN; 0.44 to 3.34 & PLUSMN; 1.31 mg C/g SOC. The addition of SF induced an immediate and short-lasting positive PE, whereas that of IF caused a gradual and long-lasting positive PE. The immediate PE was associated with abundant, substrate-derived labile C. This indicates that the early-stage PE was controlled by substrate availability. The long-lasting PE was accompanied by an accelerated decomposition of the slow C pool, reflecting that substrate stoichiometry (C:N ratio) and total C input control the later-stage PE by regulating the decomposition of the slow C pool. This can be further explained by a shift between stoichiometric decomposition and Nmining mechanisms of the PE due to changes in CUE, microbial utilization of substrate-C, and K- vs. r-strategists with substrate C:N ratios. Regarding net C budget, a smaller C accumulation induced by the SF addition corresponded to the higher substrate availability and C:N ratio due to the greater early-stage PE and lower CUE, respectively. We propose that in our studied soils: (1) substrate availability determines the early-stage PE through substrate-derived labile C; (2) substrate stoichiometry and total C input regulate the later-stage PE through microbial utilization of substrates; and (3) both substrate availability and stoichiometry control net C budget. Our study highlighted that the shift in PEs with incubation time was regulated by substrate availability, stoichiometry, and C input at the level of the C input in natural spruce forests.